The present invention relates to an image printing apparatus, such as a copying machine, a printer, and so forth, in which the electrography is employed.
Conventionally, in a thermal fixing device of the type used for a color printer or a color copying machine, the following thermal fixing method has been widely used. The machine is provided with a paper contacting part composed of two rollers, one of which is a heating roller and the surface of each of which is covered with layers of elastic material, and a non-fixed toner image carried on a sheet of printing paper is fixed by heat applied to the sheet of printing paper from the heating roller while the printing paper is grasped and transported between the two rollers. This fixing method tends to cause a problem in that some of the toner is melted by the heat from the heating roller, and a pert of the melted toner is separated from the toner image and adheres to the heating roller, resulting in what is called an offset phenomena, and the offset toner adhering to the heating roller is transferred to the pressing roller or another sheet of printing paper, so as to contaminate the surface of the pressing roller or the printing paper.
Therefore, the releasability of toner from the surface of the heating roller has been secured by applying a releasing agent to the surface of the heating roller.
Now, as a releasing agent to secure the releasability of the toner from the surface of the heating roller, various kinds of silicone oil have been used corresponding to the material used for the heating surface. For example, dimethyl silicone oil and mercapto-denatured silicone oil, including the mercapto-group as a functional group, which is disclosed in Japanese Patent Publications 41699/1984 and 6463/1989, have been applied on surface materials of silicone rubber and fluororubber, respectively.
However, if silicone rubber is used for the surface material of the heating roller and dimethyl silicone oil is used as a releasing agent in the fixing device, although a satisfactory releasability of the toner from the surface of the heating roller can be obtained with a uniformly formed oil film in the beginning, the oil film becomes nonuniform over time because the surface of the heating roller gradually becomes rough as many sheets of printing paper pass between the two rollers. As a result, the releasability of the toner from the surface of the heating roller degrades rapidly, and the offset phenomena or the winding of a sheet of printing paper around the heating roller occurs.
On the other hand, if mercapto-denatured silicone oil is used as a releasing agent, a strong oil film is formed by the action of the mercapto-group, in which the mercapto-group causes ionic-bonding with fillers of metal oxide contained in the rubber surface of the heating roller, even though the surface of the heating roller becomes rough after passing a large number of sheets of printing paper between the two rollers. Therefore, it is possible to maintain the releasability for a long time. However, the mercapto-group volatilizes as methyl-mercaptan (CH.sub.3 SH), as a result of long time heating of the rubber surface, and an odor of mercaptan is generated.
Conventionally, generation of the odor has been reduced by using oil in which the mercapto-group is decreased to as low a level as possible.
As a method of deodorizing odor generated from not only mercapto-denatured silicone oil, but also from toner or the printing papers, which is caused by the heat generated during the fixing operations, Japanese Patent Application Laid-Open 20752/1995 discloses a method of expelling odors by using a fan and a duct provided above the fixing device, and Japanese Patent Application Laid-Open 8557/1985 also discloses a method of decomposing offensive substances by using an oxide catalyst formed in a honeycomb shape in which a catalyst of alumina-Mn.sub.2 O.sub.3 --ZnO, alumina-Mn.sub.2 O.sub.3 --Fe.sub.2 O.sub.3, and so forth are used.
However, reducing the amount of the mercapto-group brings about a lowering of the ionic-bond action of the mercapto-group, in which the mercapto-group causes ionic-bonding with fillers of metal oxide contained in the rubber surface of a heating roller. Consequently, the oil film gradually becomes non-uniform and the releasability in the surface of the heating roller degrades, as the surface of the heating roller becomes rough after passing many sheets of printing paper between the two rollers. Therefore, even if mercapto-denatured silicone oil is used, it has been difficult to greatly extend the life of the fixing device.
Moreover, the above deodorizing methods disclosed in Japanese Patent Application Laid-Open 20752/1995 and Japanese Patent Application Laid-Open 8557/1985 can not decompose the odor of methyl-mercaptan enough, and so it is necessary to increase the surface area of the oxide catalyst element or to gain more time for decomposing the offensive substances by decreasing the speed of the exhaust gas passing through the oxide catalyst. However, the above-mentioned countermeasures cause an increase in the size of an electrograph apparatus or an increase in the temperature in the apparatus, which is due to the heat generated in the fixing device.